Understanding backup path calculation

It’s important to highlight that the outputs still specifically mention Local-LFA. This distinction arises because both classic LFA and TI-LFA are active in the topology in Figure 5.1. Furthermore, classic LFA inequality is met for link protection.

The following explanation details the computation and selection of the primary path and backup path:

Primary path

The primary path from P2 to P8 goes via P7, with an end-to-end path cost of 20 (P2–P7–P8), making it the shortest path.

Backup path

The following conditions are applied for the backup path.

Classic LFA

The condition for Classic LFA inequality in link protection is as follows:

Distance_opt(N, D) < Distance_opt(N, S) + Distance_opt(S, D)

Here, S = P2, D = P8, there are two neighbors, so N = P3 and P6:

• Inequality condition for P3

  S = P2, D = P8, and N = P3

  Distance_opt(P3, P8) < Distance_opt(P3, P2) + Distance_opt(P2, P8)
  20 < 10...